Nucleic acid pharmaceuticals suppress expression of pathogenic proteins by delivering functional nucleic acids into the cytoplasm, are attracting attention as pharmaceuticals of the next generation, and have been studied extensively. Carriers are indispensable since functional nucleic acids have low stability in blood and are rapidly decomposed. Carriers for delivering nucleic acids include, for example, lipid membrane structures such as virus, polymer micelle, liposome and the like.
Viral carriers are most widely used for their high expression efficiency; however, they have pathogenicity and antigenicity, and functionality cannot be imparted easily. Therefore, development of a safer non-viral carrier is required, and researches of polymer micelle, lipid membrane structure and the like are ongoing. Polymer micelle is a carrier composed of polyethylene glycol (hereinafter to be referred to as “PEG”), polyamino acid and the like, and examples of clinical development are still few.
Lipid membrane structure is a carrier composed of cationic lipid, phospholipid and the like. It is advantageous in that the composition of components constituting the carrier can be changed easily and functionality can be imparted with ease by structural modification. Plural clinical developments have been conducted so far, and it is the non-viral carrier used most generally.
To effectively deliver a functional nucleic acid into the cell by using a lipid membrane structure as a carrier, it is necessary to improve intracellular kinetics such as uptake into cells, endosomal escape capability and the like, in addition to the improvement of pharmacokinetics of lipid membrane structure such as stability in blood, tumor accumulation property and the like.
A cationic lipid which is one of the constituent components of a lipid membrane structure is used for the purpose of imparting pH responsiveness to the lipid membrane structure. By using a cationic lipid, lipid membrane structures can be stably present in the physiological environment such as blood and the like. On the other hand, under the acidic environment such as in the cell, collapse of the lipid membrane structure enables release of the drug into the cytoplasm.
Cationic lipids are roughly composed of a hydrophobic moiety and a hydrophilic moiety, and the hydrophobic moiety is a hydrophobic group such as fatty acid residue, sterol residue and the like, and the hydrophilic moiety is a cationic group such as amino group, ammonium group and the like. In particular, many structures containing two hydrophobic groups in the hydrophobic moiety and one cationic group such as amino group, ammonium group and the like in the hydrophilic moiety (two-chain type cationic lipids) are known.
As a cationic lipid to be used for a lipid membrane structure, a known compound of 1,2-Dioleoyl-3-dimethylaminopropane (hereinafter to be referred to as “DODAP”), 1,2-Dilinoleoyl-3-dimethylaminopropane (hereinafter to be referred to as “DLinDAP”) and the like can be mentioned.
The amino group contained in the cationic lipid is protonated and changes to cationic as the surrounding pH decreases, thereby imparting pH responsiveness to the lipid membrane structure.
A lipid membrane structure administered to a living organism is incorporated into endosome. It is known that early endosome moves to the vicinity of the Golgi apparatus, matures into late endosome containing a large number of endoplasmic reticula and binds to lysosome. When the late endosome and the lysosome are bound, the functional nucleic acid is decomposed by the degrading enzyme in the lysosome. Therefore, for an efficient delivery of functional nucleic acids into cells, it is necessary to release functional nucleic acids from the early endosome into the cytoplasm before the binding of late endosome and lysosome occurs.
However, non-patent document 1 maintains that a lipid membrane structure encapsulating functional nucleic acid releases the functional nucleic acid into the cytoplasm only in the stage of the early endosome and the amount of release is several percent.
As shown above, despite the technical progress in this field, the intracellular nucleic acid deliverability achieved by a lipid membrane structure using conventional cationic lipid is not fully satisfactory.